I mean, I'd say a lot had changed myself. Prior to 3E there was no clear indication they were even "reptilian", per se. They're scale-y, sure, but this is D&D and lots of things are, and they have horns, and are consistently drawn in a non-reptilian way. I can't think of a single "reptile-style" kobold prior to late 2E/early 3E. You can even see a dog-nose on the example you have from 1E, and ears! The 2E description from the MM doesn't mention them as reptilian at all, just scale-y (they lay eggs, but so do some mammals), but they have "rat-like" tails, smell, like dogs, yap like dogs, and so on.1) Actually exercise discipline over the art. There was a lot of variants in how artists did kobolds over the years, which spawned a lot of confusion. But the text descriptions in the monster books (like the 2e Monstrous Compendium Volume 1) gave them "dog-like" heads and scaly bodies. Compare the canonical 1e MM image and the canonical 3e MM image, and tell me how much changed other than art style?
Whereas in 3E, we basically have a humanoid dinosaur (procompsognathus-inspired), with tiny horns for the sake of acknowledging it's a Kobold, having lost the ears, mammalian nose (or flat face of some of the 1E/2E kobolds) and ratlike tail, and so on.
Re: Draconic I guess the big change with 5E is that they clearly decide "Okay, they're reptilian" and abandon the mammalian characteristics they previously had. Hence them speaking Draconic not their own language or Orc/Goblin. So not draconic out of the game, but solidly reptilian out of the gate.
So then given their association with dragons from Dragon Mountain, and Meepo and so on, we see the actual decision is somewhere in the 2000-2003 range as confirmed by Race of the Dragon (not sure why Mongoose's book would matter - they had no special insight did they? Or did WotC people work on it?).